Lifting device

Abstract

An electromagnetic switching valve, for which the maximum opening is set to be small, is disposed on piping between a lift cylinder and a hydraulic pump motor. A pilot check valve, for which the maximum opening is set to be larger than the electromagnetic switching valve, is disposed on piping, different from the piping, between the lift cylinder and the hydraulic pump motor. In addition, during lowering operations, first, the electromagnetic switching valve is opened, and then after the same is opened, the pilot check valve is opened after a prescribed time has passed. Thus, the shock generated when lowering an object to be raised/lowered is reduced and a fork is operated quickly.

The invention claimed is: 
     
       1. A lifting device that lifts and lowers a lifting material by supplying and discharging hydraulic oil to and from a hydraulic cylinder, the lifting device comprising:
 a hydraulic pump that supplies the hydraulic oil to the hydraulic cylinder; 
 a first oil passage that connects the hydraulic cylinder and the hydraulic pump; 
 a second oil passage that connects the hydraulic cylinder and the hydraulic pump; and 
 an opening-closing unit that opens and closes the first oil passage and the second oil passage, wherein 
 the first oil passage has a maximum oil passage area that is smaller than a maximum oil passage area of the second oil passage, 
 the first oil passage includes 
 a first portion between the hydraulic cylinder and the opening-closing unit, and 
 a second portion between the opening-closing unit and the hydraulic pump, and 
 the opening-closing unit 
 allows the hydraulic oil to flow through the first oil passage when the lifting material is lowered, and 
 after the first oil passage opens, allows the hydraulic oil to flow through the second oil passage when a first pressure difference between the first portion and the second portion decreases to a predetermined pressure difference or less, 
 wherein the opening-closing unit includes 
 a first direction control valve arranged in the first oil passage, wherein the first direction control valve switches a flow direction of the hydraulic oil in the first oil passage, and 
 a second direction control valve arranged in the second oil passage, wherein the second direction control valve switches a flow direction of the hydraulic oil in the second oil passage, 
 the maximum oil passage area of the first oil passage is determined by a maximum open degree of the first direction control valve, 
 the maximum oil passage area of the second oil passage is determined by a maximum open degree of the second direction control valve, and 
 the maximum open degree of the first direction control valve is smaller than the maximum open degree of the second direction control valve. 
 
     
     
       2. The lifting device according to  claim 1 , wherein the hydraulic oil flows from the hydraulic cylinder toward the hydraulic pump through the first and second oil passages when the first direction control valve and the second direction control valve open, thereby causing the hydraulic oil to function as driving power used for driving the hydraulic pump as a hydraulic motor so that the hydraulic motor performs a regeneration operation. 
     
     
       3. The lifting device according to  claim 1 , wherein the maximum open degree of the second direction control valve is set to be in a range of 20 to 50 times larger than the maximum open degree of the first direction control valve. 
     
     
       4. The lifting device according to  claim 1 , further comprising a measurement unit that measures a time elapsed from when the first direction control valve opens, wherein
 the opening-closing unit opens the second direction control valve when the elapsed time reaches a predetermined time. 
 
     
     
       5. The lifting device according to  claim 1 , further comprising:
 a third oil passage through which the hydraulic oil that has passed through the second direction control valve flows; and 
 a switch valve arranged in the third oil passage, wherein 
 the first direction control valve is an electromagnetic switch valve, 
 the second direction control valve is a pilot check valve including a valve body accommodated in the second direction control valve and a throttle oil passage formed in the valve body, 
 the opening-closing unit is configured to open the switch valve, 
 when the switch valve opens, the hydraulic oil is discharged from the hydraulic cylinder to the third oil passage through the throttle oil passage, which generates a second pressure difference between an inflow side and an outflow side of the throttle oil passage, and 
 the valve body moves in a direction in which the second oil passage opens in accordance with the second pressure difference. 
 
     
     
       6. The lifting device according to  claim 5 , wherein the switch valve is set to have a maximum open degree that is
 smaller than the maximum open degree of the pilot check valve, and larger than or equal to the maximum open degree of the first direction control valve.